Existing underwater hand weapons for swimmers typically use rubber bands or a variety of mechanical springs, compressed gas, and hydraulic accumulators to propel a spear. Less common are propellant fired weapons, but at least one designer has proposed a specially designed watertight gun that fires a spike with a sealed in propellant.
Even though the majority of our beautiful earth is made up of a vast expanse of never ending oceans, man over the ages has focused most of his technology development on land based inventions and has put little thought into underwater firearm technology. One area that has been overlooked is underwater firearm technology where the firearm is the same weapon used above the water level.